This invention relates in general to the field of analog and digital voice and digital data communications and particularly to expansion apparatus to enable additional subscribers to be added to an existing communication system in a simple and reliable manner.
A major problem which exists in the design, manufacture and operation of communication systems, especially those systems which are defined in the prior art as PBX Systems, (Private Branch Exchange Systems) or PABX Systems (Private Automatic Branch Exchange Systems) is the expansion of the system as the requirements of the user increase. Many companies who supply such systems provide a family of products in which the cost effectiveness of each of the products is related to the particular size of the initial system which is a function of the number lines.
As one understands, in a communication system such as in a telephone switching system, each subscriber line is associated with a terminal such as a subset and with a line circuit which will allow the terminal to interface with the communications network. In this manner as the number of lines increases, the number of terminals and line circuits increases and hence, the total cost of the system increases.
Apart from this factor is the additional factor that the original central control of the switching system such as the memory translation and other switching functions must be designed to accommodate some maximum number of lines. In many instances this maximum number may not be ascertained, as it becomes a function of the future needs of the subscriber.
Since most, if not all, systems contain some common equipment, both the cost of this equipment and the cost of the number of lines has to be paid for and amortized among the number of lines even at the very low end of the range. For example, if one designed a control and switching network to accommodate 10 lines, the network would still have to have the necessary control to implement connections between each of the 10 lines. Thus this system would have a cost based on the size of the common control as well as the number of lines. In any event, if one built a system having 100 lines, the same common control would still be necessary, but there would be an additional number of lines which would increase the total system cost. Therefore, the 10 line system essentially will have a higher cost per line than the 100 line system. Thus the art recognizes that there is a relatively high cost per line at the low end. Among the same lines, the size complexity and therefore the cost in that portion of common equipment is somewhat proportional and keyed to the expansion capabilities of the system to grow to larger sizes whereby more subscriber lines are included.
Essentially, from the above consideration, it is easily inferred that the cost effectiveness and the ability of a system to grow from very small line size (e.g. 5 to 10 lines) to the larger end of the spectrum (e.g. 10,000 lines), while maintaining cost effectiveness throughout the range is an almost incompatible requirement. The prior art implemented many approaches in an attempt to solve the growth and the cost effectiveness requirements in such provided switching systems. The most common approach has been to specify the size range and to design switching systems tailored to each one of those line sizes. For example, many manufacturers of PBX and PABX devices provide a small, a medium and a large system, and each user will purchase one of the three systems according to the number of terminals needed. This is a typical approach used by many manufacturers.
A refinement of this approach has been an attempt to keep the same basic technology and/or building blocks to maintain a family concept throughout the products covering the various ranges. Essentially, this implies that the manufacturer will provide a switching network and a common control which uses identical modules, and the modules are added to obtain small to larger switching networks. This solution has been adequate to accommodate switching networks using very few lines as well as switching networks of medium size which is between 200 to 300 lines. In any event, even the most advanced technological concepts for switching systems have evidenced a need for drastic changes within the system to allow one to make a transition from a medium size network to a large size network. Thus, in attempts to expand the system from 300 to 10,000 or more lines, one experiences the need for fundamental changes which in turn manifests itself in extremely high cost per line in implementing such a transition. A common limitation of the approaches outlined above is the fact that once a system has been installed in the field, then growth capabilities are limited by the upward bound range for which the individual product has been designed. Accordingly, when a given user has reached the saturation end of the product in use, and in order to satisfy the growth needs, the current system has to be discarded and a new investment is required which indicates increased expense to enable the user to purchase a new system for the larger range.
It is, therefore, an object of the present invention to provide an improved switching system which will enable expansion from a low order system to a high order system in a simple and efficient manner. Another object of the present invention is to provide apparatus which will enable a small switching system to accommodate an increased number of lines by the use of special junctor interfaces which will provide for a rapid and immediate expansion from a low number of lines to a higher number of lines.